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Thread: Antenna Locations

  1. #1
    Registered User
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    Jun 2005
    Chicago IL USA

    Antenna Locations

    Does anyone know of a good tutorial regarding the placement of antennas on aircraft?

    I am concerned both with the absolute placement of an antenna, as well as it's placement relative to other antennas.

    I am considering a situation where I would have 2 COM antennas, a transponder antenna, one or two GPS antenna(s), and an ELT antenna.

    I would like to place each of them where they would achieve their best performance, but I want to make sure that I place them where one would not interfere with another.


  2. #2
    Registered User wally's Avatar
    Join Date
    Mar 2004
    southwest TN.

    dont know any tutrials but


    You might take a trip to the local airport and have a look at as many airplanes as you can find.

    I know this is a simplistic approach but it is a place to start.

    I have a Cessna 150 and can tell you the comm antenna is on the top of the cabin. It is a Valcom 760 and I can communicate easily 30 miles if I have some altitude. A lot of cessnas I have seen have both comm antennas on the cabin top, about 2 feet apart.

    My transponder antenna is almost on the belly, kinda at a 45 degree angle down, just ahead of the pilot door. On or near the fuselage bottom is usually where they go.

    GPS are receive only and will not bother each other. It is a very low strength signal from the satellites so the antenna(s) do need as clear a "view" of the sky as they can have. What I have seen on cessnas and RV's is on the top behind the cabin or cockpit area. I use a hand held GPS and set it on the inst. panel top in a wood holder I made.

    My ELT is mounted behind me inside with a wire whip antenna just sticking up and curves against the cabin roof. It is impact activated and also removable from the mount, assuming I survive the crash. I hope I never have to try it out.

    The only thing I can think that will generally affect performance is the ground plane under the antenna. It will be best is it is a large sheet of metal like aluminum. For composite airplanes, some radiating strips of copper or aluminum imbedded in the structure will do. Make them at least as long as the antenna is high.

    I do like my Valcom 760 radio - good price, good service, good performance. I have a King transponder, it was in the plane when I bought it a couple of years ago and it just keeps working and working, I have an ACK encoder for altitude info tied to it (mode C) and I have had to replace it once with the latest model.

    On the big planes I work on for work (Boeing, Douglas, Airbus, G1, GII,GIII, GIV, GV), the #1 comm is usually on the top of the fuselage and the #2 is on the bottom. #3 comm is usually back on top. Dual GPS ants are on the top about a foot apart, mostly for structual reasons. The dual transponder ants. are on the bottom as well as the radar altimeter #1 and #2, 2 each. Mode S transponder ants. go one on top, one on bottom. The VOR ants are flat things usually half way up the vert. tail. LOC antenna is either under the radome (nosecone) or sometimes on the nose gear itself.

    HF antennas are sometimes made as part of the leading edge of the vert fin, a probe out the back of the tailcone, or a probe out the top of the vert. Sabreliner 65 uses a short nacelle-to-wing HF antenna. OTher models use a long wire ant. known as a clothesline ant, because that is what it looks like. The ADF ant is sometimes just a long piece of wire.

    Hope this helps just a little, it was fun for me to review some of the things I have seen over the years.

  3. #3
    Registered User Waiter's Avatar
    Join Date
    Mar 2005
    Northwestern Ohio
    A couple considerations is the aircraft construction, i.e. wood/fabric, all metal skin, glass skin.

    As Wally mentioned, stroll along the flight line. Most of the metal spam cans you see employ ground plane antennas.

    Keep these three rules in mind:
    Comm, ELT, and Transponder antennas need to be oriented up/down (vertical).

    GPS doesn't make a difference (circular)

    NAV (VOR) needs to be sideways (horizontal)

    Generally, you'll find the Comm antennas mounted on the Top and/or Bottom of the fuselage. If you have 2 comms, one on top and one on bottom is ideal, as this minimizes the interference that they will cause each other.

    The Xpndr performs best in the bottom of the fuselage, with the shortest possible coax routing you can make (6 to 10 feet) The antenna should have a clear unobstructed 360 degree view of the ground, horizon to horizon. Keep other antennas as far as you can from the Xpndr Antenna. This antenna puts out over 200 watts of energy.

    The GPS is just the opposite. It needs to be on the top, and should have a clear unobstructed 360 degree view of the sky, horizon to horizon. If you have an "unpowered" GPS antenna, you need to keep the coax routing as short as possible (under 6 feet). Coax lengths for powered antennas (they have a amplifier in the antenna) arn't as critical.

    Use a good quality 50 ohm coax, RG-400U or similiar. DO NOT USE 75 ohm coax. This is OK for your TV, but not worth a darn for your radio.

    Try to keep your antenna cable routing (especially the Transponder) seperate from other wiring. This keeps noisy electrical signals from getting into the coax, and keeps the transmitters out of the electrical wires.

    Last edited by Waiter; July 31st, 2005 at 04:14 PM.

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